Quasars at the highest known redshifts potentially offer the strongest
leverage on cosmological parameters, and the opportunity for studying
high-z QSOs has grown rapidly in recent years with the discovery of
an increasing number of sources at z > 4. Observational results for
z > 4 QSOs were reviewed at this conference by Shields; in mid-1998
there are approximately 90 such objects reported in the published literature.
Spectroscopic properties for major subsets of these sources have been
reported previously by
Schneider et
al. (1991),
Storrie-Lombardi et
al. (1996), and
Shields & Hamann (1997).

To first order, QSOs at high z appear very similar spectroscopically
to sources at lower redshift. A composite spectrum derived from
observations of 21 QSOs with z > 4 is shown in
Figure 8.
The apparent normality of the emission-line spectra of z > 4 quasars
is actually a striking feature when one considers that the lookback
time is greater than 90% of the age of the universe (for q0 =
0.5, = 0). To the extent the
emission spectra of
quasars reflect the abundances of elements like C, N, and O, one might
expect to see changes as a result of the chemical evolution expected in
the host galaxies of quasars over cosmological time scales. In general,
however, order-of-magnitude differences in abundances lead to much smaller
variations in BLR line strengths in photoionization models, due to
thermostatic feedback effects. An important exception may be present
in the N V 1240 emission from
QSOs, which may be sensitive to
secondary nitrogen enrichment (Section 3.1),
and does not display a BE.
The N V feature remains relatively strong at z > 4, pointing to
rapid early enrichment in the quasar nucleus environs (see
Hamann & Ferland 1992,
1993
for details). As discussed by
Shields & Hamann (1997),
these sources also show evidence of elevated O I
1304 emission. This feature
forms primarily via fluorescence
pumped by H Ly line
coincidence, and is consequently expected to scale
with the O/H ratio, although factors other than metallicity
may also affect its strength.

Quasars detected at z > 4 are almost invariably high-luminosity
objects, and provide useful data points at high L for inclusion in
the Baldwin diagrams. A summary of results to date is shown in
Figure 9 for the C IV line, with data taken from
Shields et al. (1998)
for a sample of objects that are largely selected on the
basis of optical colors (filled points), and from
Schneider et al. (1991)
for grism-selected objects (x). The dotted line
represents the fit obtained to the BE by
OPG for quasars
at lower z.
Schneider et al. noted a tendency for many of their sources to fall
below the extrapolated BE in this diagram; with the addition of the
Shields et al. points, the results appear consistent with the
low-redshift fit, accompanied by the customary degree of scatter.

Figure 9. Baldwin diagram for quasars at
z > 4. The filled points represent MMT observations from
Shields et al. (1998),
while the x's represent data from
Schneider et
al. (1991).
The C IV BE reported by
OPG, based on
measurements at lower redshift, is shown by the dotted line.

Selection effects remain a point of concern in comparing the z > 4
findings with low-redshift results. The color-selected objects in
particular are sensitive to inclusion of high-W objects that
are preferentially found in magnitude-limited surveys, due to the
added flux contributed by the emission line (typically Ly
observed in the R bandpass (8); see
Kennefick et al. 1995
for quantitative details). This bias may contribute
to a weak tendency for the color-selected sources to exhibit larger
equivalent widths than are found for the grism-selected objects in
Figure 9(9).